The respiratory system consist of the nasal cavity, pharynx, trachea, bronchi and the lungs. Cartilage in the walls of the trachea and bronchi develops an on-going process of calcification, causing them to become more rigid with age. Smooth-muscle fibers in the walls of the bronchioles tend to be replaced with noncontractable or unstretchable fibrous connective tissue.
The alveoli (lung's air sacs) enlarge, weaken and become less efficient. Capillary beds diminish, lowering blood flow and oxygen exchange. These changes along with a reduction in elasticity of the lungs and the wall of the thorax (muscle structure of the diaphragm), cause a gradual decrease in maximal ventilation (maximum breathing capacity) and in vital capacity (the inspiration). The decline in aerobic capacity may be contributed to an age-related decrease in muscle mass, the ability to redirect blood flow from organs to working muscles, and the ability of muscle cells to extract and use oxygen from the blood.